Fix retryAfter delay, add dynamic delete delay, update default delays

-Fix Discord API reporting too short retryAfter (assumed need to convert to seconds, x3 for margin) -Fix/add dynamic delete delay mechanism: +150ms every time rate limited, -50ms every 1000 deleted messages (about 22min at 1300ms) -Update default delays to better reflect Discord rate limit

Update deletecord.user.js

Merge pull request #5 from larryqiann/delay-fix

Fix retryAfter delay, add dynamic delete delay, update default delays

Skip deleting messages on insufficient permissions

Merge pull request #1 from peesock/main

Skip deleting messages on insufficient permissions

update description and name